Force measurement device

ABSTRACT

The present invention relates to a force measurement device that includes mechanical amplification of linear elastic deformation along the axis of loading by estimating the quasi-linear incremental displacement between two points on arcs inscribed due to angular movements of a pair of cantilever arms located on opposite quadrants on a closed contour load sensing element to improve the sensitivity, and hence the resolution, of the force measurement device.

FIELD OF INVENTION

The invention relates to a force measurement device typically used inmaterial and structural test systems for precise measurement of staticand dynamic loads.

BACKGROUND OF THE INVENTION

A typical force measurement device, also known as load cell, consists ofa load sensing element which deforms elastically in proportion toapplied load. This in turn changes the electrical properties ofsensitive elements such as strain gauges bonded to the load sensingelement. Such a change in electrical properties can be correlated to theapplied load. Force transducers, in industrial applications, typicallyuse this principle.

The sensitivity of a force measurement device is defined as the ratio ofthe change that can be measured in the physical parameter to thesmallest change in the force actually applied. The higher thesensitivity, the better the resolution of the force measurement deviceis. The sensitivity of the force measurement device is improved eitherby mechanical amplification of elastic deformation or electricalamplification of the electrical property like voltage or current.

Force measurement devices are typically of two types: (a) proving ringbased, and (b) strain-gauge based. In the proving ring based load cells,the linear elastic deformation, along the loading axis of the ringshaped force sensing element, is measured directly (without anymechanical amplification) to indicate the applied force. The sensitivityof the force measurement device is limited by the sensitivity of thedisplacement sensor, typically linear variable differential transformer(LVDT), used to measure the elastic deformation. In the strain-gaugebased force measurement devices, a set of calibrated foil resistancestrain-gauges are mounted in a particular pattern on flexural arms topick up compressive or tensile strain. These strain-gauges are arrangedto form an electrical circuit called full Wheatstone's bridge, theoutput voltage of which is correlated to the force applied to the loadsensing element. In these prior art load cells neither the elasticdeformation nor the electrical property is amplified but the outputelectrical signal, which is typically in the order of mV (millivolts),is amplified to V (volts) via additional electronic signal conditioningboards. However, the amplification of the electrical signal results inhigh noise-to-signal ratio. The better reported resolution of these loadcells is in the order of 0.02-0.05% of the full scale. Suchamplification of electrical signal involves use of analog electronics,such as analog-to-digital converter, signal conditioning hardware, etc.

SUMMARY OF THE INVENTION

The present invention discloses a force measurement device that includesmechanical amplification of linear elastic deformation along the axis ofloading by estimating the quasi-linear incremental displacement betweentwo points on arcs inscribed due to angular movements of a pair ofcantilever arms located on opposite quadrants on a closed contour loadsensing element to improve the sensitivity, and hence the resolution, ofthe force measurement device.

The force measurement device according to the first aspect of theinvention includes a closed contour load sensing element that isaxisymmetric along the axis of loading (y-axis) and the axis normal toit (x-axis). The device includes a first cantilever arm mounted on thecontour in one of the quadrants, and a second cantilever arm mounted onthe contour in the opposite quadrant of the contour, where free ends ofthe first and second cantilever arms are positioned inward of thecontour such that the arms are perpendicular to an arc at a fixed end onthe contour. Deformation caused by applied load causes both the arms toinfinitesimally rotate about their fixed point which results intoquasi-linear movement of the free ends of the cantilever arms to enablemechanical amplification of the deformation along the axis of loading.

In another aspect of the invention, the first and second cantilever armsare respectively located at an angle α and α+180 with respect to thex-axis such that angular movement of the arms about their fixed end ismaximum for any applied load.

In another aspect of the invention, at the free end of the firstcantilever arm a linear digital encoder is mounted, and at the free endof the second cantilever arm an encoder scale is mounted such that thelinear digital encoder and the encoder scale are parallel and oppositeto each other and lie along movement of the free ends of the arms.

In a preferred aspect, under compression load, the first and secondcantilever arms turn counterclockwise about their fixed end by an angle−β/2, and under tension load, the cantilever arms turn clockwise abouttheir fixed end by an angle β/2.

In another aspect, the first and second cantilever arms have lengths,such that, an imaginary line, passing through free ends of the first andsecond cantilever arms, also passes through the center of the contour.

In another aspect, under applied load and corresponding deformation freeends make quasi-linear movements on imaginary arcs, these quasi-linearangular movements lead to formation of an imaginary line which enables amechanical amplification of four times the deformation.

The force measurement device according to another embodiment of theinvention comprises a closed contour load sensing element axisymmetricalong both loading axis and axis normal to the loading axis; the first,second, third and fourth cantilevers arms are mounted on the contourrespectively at angles α, 180+α, 180−α and 360−α with respect to x-axis,where the free ends of all the cantilever arms being inward of thecontour such that the arms are perpendicular to an arc at a fixed end onthe contour; wherein deformation caused by applied load effectsquasi-linear movement of the free ends of the cantilever arms to causemechanical amplification of the deformation along the axis of loading.

In a preferred aspect, the angle α with respect to x-axis of the contouris chosen such that the angular movement of each of the cantilever armsabout its fixed end is maximum for any applied load.

In a preferred aspect, under compression load, the first and secondcantilever arms turn counterclockwise and the third and fourthcantilever arms turn clockwise and under tension load, the first andsecond cantilever arms turn clockwise and the third and fourthcantilever arms turn counterclockwise.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein the showings are for the purposeof illustrating a possible embodiment of the invention only, and not forthe purpose of limiting the same,

FIG. 1 shows the schematic of a typical force measurement device;

FIG. 2 shows the principle of operation of force measurement deviceaccording to a first embodiment the invention;

FIG. 3 shows the exploded view of the force measurement device based onthe first embodiment;

FIG. 4 shows another view of the force measurement device based on thefirst embodiment;

FIG. 5 shows the exploded view of the sensor mounting arrangement;

FIG. 6 shows the installation and operation of the load cell based onthe present invention on a servo controlled material test system;

FIG. 7 shows the graphical representation of the comparison betweenaccuracy of digital load cell of the present invention and that of loadcell based on strain gauges;

FIG. 8 shows the principle of operation of force measurement deviceaccording to a second embodiment of the invention;

FIG. 9 shows the force measurement device according to the secondembodiment with arrangement for measuring a pair of mechanicallyamplified displacements; and

FIG. 10 shows the exploded view of the force measurement deviceaccording to the second embodiment with arrangement for measuring a pairof mechanically amplified displacements.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The schematic of a typical force measurement device that works on theprinciple of electrical amplification is shown in FIG. 1. The forceapplied on a load sensing element causes an elastic deformation of theelement which would change the electric properties of, say, a straingauge bonded with the element. The sensitivity of the force measurementdevice, defined as the ratio of the measureable change in the output(physical parameter) to the smallest change in the input (the appliedforce).

The present invention, as will be described further, provides a forcemeasurement device that works on the principle of mechanicalamplification of linear elastic deformation along the axis of loading.

The principle of operation of the force measurement device according toa first embodiment the invention will be explained with reference toFIG. 2. The force measurement device includes a stiff load sensingelement having a closed contour (5) which is axisymmetric along the axisof loading and the axis normal to it. According to this embodiment, theaxis of loading will be referred to as the y-axis and the axis normal toit will be referred to as the x-axis. The first cantilever arm (1) ismounted on the contour (5) in one of the four quadrants of the contour,and the second cantilever arm (2) is mounted on the contour in theopposite quadrant of the contour (5) with free ends of the arms (1, 2)being inward of the contour (5), such that each arm is perpendicular tothe arc of the contour at its fixed end. The arms (1, 2) are located atspecially selected angle with respect to the x-axis (normal to theloading axis) such that they undergo maximum angular movement abouttheir fixed end for the same amount of contour displacement alongloading direction.

The free ends of the arms (1, 2) inscribe arcs as the loading is varied.If contour dimension R=min(a, b) and elastic deformation δ/2(corresponding to load, P) of the load sensing element along the axis ofloading such that R>>δ/2, then the incremental quasi-linear distance Δ/2moved by the free ends of the arms (1, 2) are collinear and Δ=28. Now ifthe applied load is correlated to this mechanically amplifieddisplacement Δ (resulting due to angular movement of the arms) ratherthan the elastic deformation δ, the sensitivity of the force measurementdevice is twice the force measurement device based on the prior artproving ring.

According to FIG. 2, the load sensing element is considered to be closedcontour with semi-major axis a and semi-minor axis b, and is acted uponby tensile or compressive deflection along the y-direction. The contourshape remains undeformed when no force is applied and such a contourserves as a reference. The deformed contours refer to tensile andcompressive deflections. Application of fully reversedtension-compression load of amplitude P causes a deformation ofamplitude δ/2 along y-direction. Rigid cantilever arms (1, 2) arerepresented as n₁m₁ and n₂m₂ (of equal lengths) in FIG. 2. The first arm(1) is located and fixed at n₁ and second arm (2) is located and fixedat n₂. The arms are free to move at other ends m₁ and m₂. The armlengths are such that the line l₁l₂, passing through m₁ and m₂, alsopasses through the origin, i.e., center of the contour. The arms n₁m₁and n₂m₂ are respectively located on the contour at angles α degrees and180+α degrees with respect to x-axis. These arms are perpendicular tothe arc on the contour and remain so even under deformed conditions.During cyclic loading these arms rotate respectively about n₁ and n₂such that the free ends m₁ and m₂ move on the arcs r₁ and r₂respectively. During the excursion of loading:

(a) the arms are parallel at no-load condition, i.e., at P=0.0 and δ=0;under this condition: the distance L=m₁m₂=L₀,(b) the arms turn counterclockwise by an angle −β/2 under compressionload P and deformation −δ/2; under this condition the distanceL=m₁m₂=L_(c)=L₀+Δ,(c) the arms turn clockwise by an angle β/2 under tension load P anddeformation −δ/2; under this condition the distance L=m₁m₂=L_(t)=L₀−Δ.

The angle α is chosen such that angular movement of the arms β/2 ismaximum for any applied load P. If the contour dimensions (a, b) andarm's length (S) are such that S, a, b>>δ then the points m₁ and m₂ makequasi-linear movements, corresponding to angular movements β/2, on theline l₁l₂ and it can be mathematically shown that:

Δ=L _(t) −L ₀ =L _(c) −L ₀≅2δ.

Following this, at any point of loading, the linearized deformationalong the line l₁l₂ is Δ_(i)≅2δ_(i). This implies that for an elasticdeformation of δ/2 corresponding to applied load P along y-axis, thereare two points m₁ and m₂ such that distance between them is almostincremented by 2δ, that is an amplification of 4 times.

EXAMPLE

Below is an example for mechanical amplification of deformation alongthe axis of loading for load sensing element with closed contour ofelliptical shape.

Given,

a=55.0 mm, b=50.0 mm, S=52.25 mm, and δ/2=2.5×10⁻⁵ mm

Results,

β/2=0.0001° and Δ/2=5.0×10⁻⁵ mm

This mechanically amplified deformation is correlated with applied loadalong y-axis. The force measurement device built around this will haveresolution doubly better than the one based on conventional provingring.

An exploded view of an exemplary force measurement device of capacity 25KN based on the present invention is shown in FIG. 3. The forcemeasurement device includes an axisymmetric closed contour (5),displacement sensor, i.e., digital encoder (6) and rigid cantilever arms(1, 2) fixed on the contour and free to rotate about their fixed end onthe contour. The closed contour (5) of the load sensing element looksalmost like an ellipse as it is formed by connecting two semicircles,located as mirror image at equal distance apart from the vertical line.At the free end of the first arm (1) a linear encoder (6) is mounted andat the free end of the second arm (2) an encoder scale (9) is mountedsuch that the encoder (6) and scale (9) are parallel and opposite toeach other and lie along movement of the free ends of the arms as shownin FIGS. 3 and 4. The closed contour (5) is housed within a back coverplate (3) and a front cover plate (8) using fasteners (4, 7). FIG. 5shows the exploded view of the encoder (6) and scale (4) mountingarrangement using the first and second cantilever arms (1, 2).

The mounting and installation of the force measurement device, based onthe current invention, is demonstrated on a servo controlledelectromechanical based portable material test system, typically knownas plug and play NANO System by BiSS-ITW. The arrangement is shown inFIG. 6 which includes the force measurement device (20) according to theinvention, top and bottom grips (22, 26), the test specimen (24),actuator piston (28) and crack opening displacement (COD) gauge (30).

FIG. 7 shows the graphical form of the comparison of the accuracy of theforce measurement using the device based on the present invention(digital load cell) with that obtained from the load cell based on thestrain gauges (analog load cell). The tabulated values of the graphicalform are given below in Table 1.

TABLE 1 % Error P (kN) Analog Load Cell Digital Load Cell 2.5000 0.04000.0040 5.0000 0.0200 0.0020 7.5000 0.0133 0.0013 10.0000 0.0100 0.001012.5000 0.0080 0.0008 15.0000 0.0067 0.0007 17.5000 0.0057 0.000620.0000 0.0050 0.0005 22.5000 0.0044 0.0004 25.0000 0.0040 0.0004

As a result of using the mechanical amplification of the linear elasticdeformation of the contour along the axis of loading and measuring itwith a high resolution encoder, the force measurement device based onthe present invention provides resolution of the force measurement about10 times more accurate than that measured from the load cell based onstrain gauges.

In another embodiment of the invention, the force measurement device, asan outcome of mechanical amplification of linear elastic deformationalong the axis of loading, is that the measured force takes into accountany misalignment in the axis of loading by measuring two sets ofmechanically amplified displacements corresponding to the elasticdeformation in the axis of loading. The principle behind this embodimentis illustrated in FIG. 8 and the design of the load cell based on thisprinciple is shown in FIG. 9 and FIG. 10.

FIG. 8 shows a closed contour load sensing element (15) axisymmetricalong a loading axis (y-axis) and an axis normal to the loading axis(x-axis). The cantilever arms first (11), second (12), third (13) andfourth (14) are mounted on the contour (15) respectively at angles,180+α, 180−α and 360−α with respect to x-axis.

The first and second cantilever arms (11, 12) form the first pair andthe third and fourth cantilever arms (13, 14) form the second pair ofcantilever arms. The free ends of the first, second, third and fourthcantilever arms (11, 12, 13, 14) are inward of the contour (15) suchthat each arm is perpendicular to the arc at the fixed end on thecontour.

The closed contour load sensing element has a semi-major axis a andsemi-minor axis b, and is acted upon by tensile or compressivedeflection along the y-direction. The contour shape remains undeformedwhen no force is applied and such a contour serves as a reference. Thedeformed contours refer to tensile and compressive deflections.Application of fully reversed tension-compression load of amplitude Pcauses a deformation of amplitude δ/2 along y-direction. The first pairof cantilever arms (11, 12) are represented as n₁m₁ and n₂m₂ (of equallengths) and the second pair of cantilever arms (13, 14) are representedas n₃m₃ and n₄m₄ located respectively at angles 180−α and 360−α degrees.The free ends m₃ and m₄ of the third and fourth cantilever arms (13, 14)make quasi linear movement Δ₂/2 on the line l₂, as free ends m₁ and m₂of the first and second cantilevers n₁m₁ and n₂m₂ make quasi-linearmovement Δ₁/2 on the line l₁, corresponding to the vertical deflectionδ/2 of the closed contour (15). During cyclic loading, the first andsecond arms (11, 12) rotate respectively about n₁ and n₂ such that thefree ends m₁ and m₂ move on the arcs r₁ and r₂ respectively. Third andfourth arms (13, 14) rotate respectively about n₃ and n₄ such that thefree ends m₃ and m₄ move on the arcs r₃ and r₄ respectively Arms n₃m₃and n₄m₄ rotate clockwise about n₃ and n₄, and arms n₁m₁ and n₂m₂ rotatecounterclockwise about n₁ and n₂ when the closed contour is compressedvertically by δ/2. Here it can be shown that:

Δ₁≅Δ₂, and

Δ=(Δ₁+Δ₂)/2

The averaged displacement derived above can be correlated to load Pcorresponding to vertical deflection δ/2 of the closed contour (15).Thus, any deviation in the readouts due to misaligned load can beavoided.

The mounting of the cantilever arms (11, 12, 13, 14) in the contourshaped (15) load sensing element is shown in FIGS. 9 and 10. Thus, themechanical amplification along the axis of loading helps in designingload cells with improved sensitivity, resolution, and accuracy.

The implementation of the force measurement device according to theabove embodiments, along with the use of high resolution encoder, hasnumerous advantages. The set-up is a fully digital load cell without theneed of any analog electronic device thereby eliminating electronichardware devices like analog-to-digital converter, signal conditioninghardware etc.

The transducer response is sensitive to contour shape change andinsensitive to size change. Therefore, the device according to theinvention is immune to drift with temperature. As long as thetemperature is uniform across the device, the contour shape will notchange. Therefore, force readout will also remain unchanged.

Further, the mechanical amplification of the elastic deformation resultsin a strictly or highly linear relationship between the force andamplified displacement since the amplified displacement is quasi-lineardue to infinitesimal angular movement of the order of 1×10⁻⁴ degrees.

The foregoing description shows and describes preferred embodiments ofthe present invention. It should be appreciated that this embodiment isdescribed for purpose of illustration only, and that numerousalterations and modifications may be practiced by those skilled in theart without departing from the spirit and scope of the invention. It isintended that all such modifications and alterations be included insofaras they come within the scope of the invention as claimed or theequivalents thereof.

1-14. (canceled)
 15. A force measurement device comprising: a closedcontour load sensing element axisymmetric along a loading axis and anaxis normal to the loading axis, the contour having an ellipticalprofile; a first cantilever arm having a first fixed end mounted on thecontour in a first quadrant of the contour, and a second cantilever armhaving a second fixed end mounted in a second quadrant of the contouropposite the first quadrant, where free ends of both the first andsecond cantilever arms are inward of the contour such that the arms areperpendicular to an arc at the respective fixed end on the contour;wherein deformation (δ/2) caused by applied load (P) causes quasi-linearmovement of the free ends of the cantilever arms to enable mechanicalamplification of the deformation along the axis of loading; and wherein,under compression load, the first and second cantilever arms turncounterclockwise by an angle β/2.
 16. The force measurement deviceaccording to claim 15, wherein the first cantilever arm is located at anangle α and the second cantilever arm is located at angle α+180 suchthat angular movement of the arms about their fixed end is maximum forany applied load.
 17. The force measurement device according to claim15, wherein at the free end of the first cantilever arm a linear digitalencoder is mounted.
 18. The force measurement device according to claim17, wherein at the free end of the second cantilever arm an encoderscale is mounted such that the linear digital encoder and the encoderscale are parallel and opposite to each other and lie along movement ofthe free ends of the arms.
 19. The force measurement device according toclaim 15, wherein the first and second cantilever arms have equallengths, such that, an imaginary line (l₁l₂), passing through free ends(m₁,m₂) of the first and second cantilever arms, also passes through thecenter of the contour.
 20. The force measurement device according toclaim 15, wherein the closed contour of the load sensing element has anelliptical profile.
 21. The force measurement device according to claim15, wherein the closed contour of the load sensing element has anaxisymmetric closed contour profile.
 22. The force measurement deviceaccording to claim 15, wherein the load sensing element has a loadcapacity of 25 Kilo-Newtons.
 23. The force measurement device accordingto claim 15, further comprising: a front cover plate; and a back coverplate, wherein the closed contour load sensing element is housed betweenthe front cover plate and the back cover plate via a plurality offasteners.
 24. A force measurement device comprising: a closed contourload sensing element axisymmetric along a loading axis and an axisnormal to the loading axis, the contour having an elliptical profile; afirst cantilever arm having a first fixed end mounted on the contour ina first quadrant of the contour, and a second cantilever arm having asecond fixed end mounted in a second quadrant of the contour oppositethe first quadrant, where free ends of both the first and secondcantilever arms are inward of the contour such that the arms areperpendicular to an arc at the respective fixed end on the contour;wherein deformation (δ/2) caused by applied load (P) causes quasi-linearmovement of the free ends of the cantilever arms to enable mechanicalamplification of the deformation along the axis of loading; and wherein,under tension load, the cantilever arms turn clockwise by an angle β/2.25. The force measurement device according to claim 24, wherein thefirst cantilever arm is located at an angle α and the second cantileverarm is located at angle α+180 such that angular movement of the armsabout their fixed end is maximum for any applied load.
 26. The forcemeasurement device according to claim 24, wherein at the free end of thefirst cantilever arm a linear digital encoder is mounted.
 27. The forcemeasurement device according to claim 26, wherein at the free end of thesecond cantilever arm an encoder scale is mounted such that the lineardigital encoder and the encoder scale are parallel and opposite to eachother and lie along movement of the free ends of the arms.
 28. The forcemeasurement device according to claim 24, wherein the first and secondcantilever arms have equal lengths, such that, an imaginary line (l₁l₂),passing through free ends (m₁,m₂) of the first and second cantileverarms, also passes through the center of the contour.
 29. The forcemeasurement device according to claim 24, wherein the closed contour ofthe load sensing element has an elliptical profile.
 30. The forcemeasurement device according to claim 24, wherein the closed contour ofthe load sensing element has an axisymmetric closed contour profile. 31.The force measurement device according to claim 24, wherein the loadsensing element has a load capacity of 25 Kilo-Newtons.
 32. The forcemeasurement device according to claim 24, further comprising: a frontcover plate; and a back cover plate, wherein the closed contour loadsensing element is housed between the front cover plate and the backcover plate via a plurality of fasteners.
 33. A force measurement devicecomprising: a closed contour load sensing element axisymmetric along aloading axis and an axis normal to the loading axis, the contour havingan elliptical profile; a first cantilever arm having a first fixed endmounted on the contour in a first quadrant of the contour, and a secondcantilever arm having a second fixed end mounted in a second quadrant ofthe contour opposite the first quadrant, where free ends of both thefirst and second cantilever arms are inward of the contour such that thearms are perpendicular to an arc at the respective fixed end on thecontour; wherein deformation (δ/2) caused by applied load (P) causesquasi-linear movement of the free ends of the cantilever arms to enablemechanical amplification of the deformation along the axis of loading;wherein the first and second cantilever arms have equal lengths, suchthat, an imaginary line (l₁l₂), passing through free ends (m₁,m₂) of thefirst and second cantilever arms, also passes through the center of thecontour; and wherein under the applied load P and correspondingdeformation β/2 the free ends (m₁,m₂) of the arms respectively makequasi-linear movement on imaginary arcs (r₁,r₂); these quasi-linearangular movements lead to formation of an imaginary line (l₁l₂) whichenables a mechanical amplification (Δ) of four times the deformation(β/2).
 34. The force measurement device according to claim 33, whereinthe closed contour of the load sensing element has an ellipticalprofile.